1. Field of the Invention
The invention relates to a sheet separator for an automatic document feeder, and more particularly to a sheet separator using two friction rollers to separate a sheet from a stack and then feed the sheet.
2. Description of the Related Art
An automatic document feeder may be used in an image input/output apparatus such as a scanner, a multi-function peripheral, a copier, or a printer. In order to feed the sheets one by one, the automatic document feeder must be equipped with a sheet separator so as to avoid error operations of feeding multiple sheets simultaneously.
FIG. 1 is a schematic side view showing a sheet separator for a conventional automatic document feeder. FIG. 2 is a schematic front view showing the sheet separator for the automatic document feeder (ADF) of FIG. 1. As shown in FIGS. 1 and 2, an ideal design of the conventional sheet separator is ideally configured to feed a plurality of sheets 120 one by one. The sheet separator includes a friction roller (also referred to as ADF roller). 110, and a friction pad 112 rotatable about a shaft 114. When the sheet 120 has not been fed, the friction pad 112 is always in contact with the stationary or rotating friction roller 110 and is thus worn out. When the outmost sheet 120 of FIG. 1 passes between the friction pad 112 and the friction roller 110, the sheet under the outmost sheet 120 may be stopped by the friction pad 112, and the sheet-separating operation may be achieved.
However, the sheet separator in the above-mentioned prior art has the following problems.
1. Since the friction pad 112 and the friction roller 110 are directly in sliding friction contact, the friction pad 112 and the friction roller 110 tend to be worn out, and the friction pad 112 that is somewhat worn out tends to cause errors in sheet-separating operations.
2. Since the friction pad 112 and the friction roller 110 are in sliding contact with each other, a motor has to provide a larger torque to drive the friction roller 110.
FIG. 3 is a schematic side view showing another sheet separator for a conventional automatic document feeder. FIGS. 4A and 4B are a pictorial view and a side view showing the friction roller of FIG. 3, respectively. As shown in FIGS. 3, 4A and 4B, the sheet separator includes a separating roller 200 and a friction roller 210. The separating roller 200 is driven to rotate by a second shaft 202, and drives the friction roller 210 to rotate using the friction force. When a sheet 230 enters the path between the two rollers, the friction roller 210 does not rotate to stop other sheets from entering the path. At this time, the sheet 230 can slide relative to the friction roller 210, but cannot slide relative to the separating roller 200. The above-mentioned phenomenon is caused by the special design for the friction roller 210. The friction roller 210 is composed of a hollow column 211, a spring 212, a friction plate 213, a fixing member 214 and two C-rings 215.
The two C-rings 215 are fastened to two ends of a first shaft 220 to provide a compression force to the spring 212 along an axial direction of the first shaft 220, and thus to provide a friction force to the friction surface 216 between the friction plate 213 and the fixing member 214, thereby stopping the rotation of the hollow column 211 fixed to the fixing member 214.
In the above-mentioned prior art, since the pushing mechanism constituted by a lot of parts between the two C-rings 215 is difficult to be manufactured and assembled, the manufacturing and assembling costs cannot be effectively reduced. In addition, the C-rings 215 have to additionally exert an axial compression force on the first shaft 220 of the friction roller 210, thereby complicating the design.
Consequently, it is an important subject of the invention to provide a sheet separator capable of overcoming the above-mentioned problems.